Adhesion promoting composition for metal leadframes

ABSTRACT

A process for increasing the adhesion of a polymeric material to a metal surface, the process comprising contacting the metal surface with an adhesion promoting composition comprising: 1) an oxidizer; 2) an inorganic acid; 3) a corrosion inhibitor; and 4) an organic phosphonate; and thereafter b) bonding the polymeric material to the metal surface. The organic phosphonate aids in stabilizing the oxidizer and organic components present in the bath and prevents decomposition of the components, thereby increasing the working life of the bath, especially when used with copper alloys having a high iron content.

FIELD OF THE INVENTION

The present invention relates generally to an improved adhesionpromoting composition for metal substrates, such as copper and copperalloy substrates.

BACKGROUND OF THE INVENTION

Microelectronic circuits such as silicon semiconductor integratedcircuits (IC) and hybrid microelectronic circuits require a packagewhich both encases the circuit and provides electrical interconnectionto external circuitry. A leadframe is one common means of electricalinterconnection. The leadframe is formed from a strip of electricallyconductive metal which is formed into a plurality of leads. The innerlead ends of the leadframe approach the integrated circuit device fromone or more sides and are electrically interconnected to the device bythin bond wires. The outer lead ends of the leadframe are electricallyinterconnected to external circuitry such as a printed circuit board.

A leadframe can contain several units, depending on the size of theindividual components and the lead frame size. The process of building alead frame chip package involves forming the metal lead frame,selectively silver plating the die attach and wirebond pads, attachingthe die (chip) to the lead frame with an epoxy adhesive, connecting thedie to the leads of the metal frame by wirebonding conductive (e.g.,gold) wires between the die and leads, encapsulating the package in anepoxy molding compound and then separating the individual units from thelead frame (singulation). These process steps may require hightemperatures and physical handling, all of which produce internalstresses on the package as it is assembled. In addition, the integrityof the package depends on adhesion of the various components to thesurface to which they are attached.

In addition, the electronics industry is moving towards the use oflead-free plating material for plating IC leadframes. Combinations suchas nickel-palladium-gold (NiPdAu) are replacing prior art tin-lead(SnPb) solders. Because of the use of these new lead-free platingmaterials, a number of new temperature/moisture related issues havearisen, most of them linked to the elevated reflow temperatures requiredto form acceptable lead-free solder joints. Eutectic solder reflowtemperatures are typically in the 200° C. to 215° C. range, while thenew lead-free solders require reflowing temperatures in the 240° C. to250° C. range. This 30-40° C. increase in reflow temperature forlead-free solders has had a wide ranging effect throughout theelectronics production supply chain and has affected virtually everymaterial and component used to manufacture a printed circuit boardassembly (PCBA).

Moisture content in IC leadframe packages has been a defect andreliability issue for quite some time but recently, as assemblersincrease reflow temperatures to accommodate lead-free solder reflowrequirements, the matter of moisture related defects has been compoundedsince the vapor pressures in a package have increased exponentially withthese elevated reflow temperatures.

To protect the device from moisture and mechanical damage, the innerlead ends and the device are encapsulated. Encapsulation may be by amolding resin which surrounds both the inner leads and the integratedcircuit device. Alternatively, discrete base and cover components definea cavity. When the base and cover are bonded together, the inner leadends and integrated circuit device are encapsulated within that cavity.

Vapor pressure, caused by the varying degrees of moisture within a givenpackage turning to water vapor, builds during reflow and once it reachesa pressure level exceeding the chemical and/or mechanical bond strengthsof the materials used to form the IC leadframe package, it releasesthrough the path of least resistance.

Good adhesion of the inner lead ends to the molding resin is required toprevent the egress of water along the leads. Moisture can corrode thebond wires and the integrated circuit device. Additionally, the moistureaccumulates inside the package. When heated, the moisture expands assteam, swelling and potentially cracking the package (i.e.,“popcorning”). When discrete base and cover components are utilized, themid-portion of the leadframe is bonded to both the base and to the coverwith a thermosetting epoxy or a low temperature sealing glass. Goodadhesion is required to prevent the egress of moisture.

Popcorn cracks, or “popcorning” as it is often called, earned thisinteresting moniker via the audible popping sound made when the vaporpressure in the component is released. Popcorn cracks are almost alwayslarge and can cause significant internal damage within the package. Inmany cases the defects are so large that electrical interconnects arebroken, rendering the component non functional. This effect has both agood side and a bad side. The good side is that the defects can likelybe identified during in-circuit testing (ICT) testing after assembly anddo not escape to the field. The negative side is that a typical PCBAprocess requires expensive rework processes to replace the component.

A less volatile, but an equally concerning IC leadframe defect is called“delamination.” This defect has the same root cause as popcorning, i.e.vapor pressure, but has more potential for latent defects and fieldfailures. When delamination occurs, the vapor pressure causes aseparation between the components material interfaces in its attempt toescape. For various reasons, the mechanical separation of interfaces hasa lower release level thus the physical damage to the electricalinterconnects within the package is minimal. Therein lies theproblem—the IC leadframe package functions properly after assembly butnow contains pathways by which moisture can enter the package, which canultimately result in corrosion, metal migration, and other electricalproblems, and eventually end with device failure. Environmental factorssuch as temperature, humidity levels, and atmospheric contaminants alsoplay a role in determining when the component will fail. This long termreliability issue can prove quite costly when a device fails in thefield and requires PCBA replacement, failure analysis, customerdissatisfaction, and other negative consequences.

Both popcorning and delamination are defects in IC leadframe packagesbut their respective effect on the packages can vary significantly.Assembly techniques and choices of materials play a role in determiningthe severity and frequency of the popcorning or delamination.

In addition, many manufacturers are requiring that IC leadframecomponents pass the JEDEC MSL-1 test criteria. MSL is an acronym formoisture sensitivity level and has eight different levels, MSL-1 beingthe highest performance level, and meaning that the package is immune topopcorning, regardless of exposure to moisture. JEDEC is an organizationthat develops and sets test, measurement, and performance standards forthe component industry, and the various classifications used todetermine a component's MSL correlate with end use and customerrequirements.

Thus, one important aspect of packaged integrated circuits is their MSL,which reflects the degree to which the integrated circuits resistmoisture induced stresses that can cause failure. The MSL of a packagedinterconnect depends in part on the quality of the seal at the interfacebetween the plastic encapsulating mold compound and the metallic leadsthat extend from the package.

One area that IC leadframe manufacturers have focused on is theinterfacial bond between the metal surfaces of the leadframe and theplastic encapsulating mold material used in the devices. This interfaceis a common area of failure during assembly because moisture in thepackage is superheated during reflow.

Various adhesion promoting compositions have been developed that enhancethe bond between the metal leadframe and the plastic encapsulating moldmaterial.

However, it is has been found that when these adhesion promotingcompositions are used with certain copper alloys, such as those having ahigh iron content, the iron content in the working bath rises, whichthen causes a significant increase in decomposition of certaincomponents present in the bath which limits the useful working life ofthe bath. Thus, it would be desirable to improve the stability of suchadhesion promoting compositions to avoid these problems.

SUMMARY OF THE INVENTION

It is an object of the present invention to prevent popcorning anddelamination of a moisture sensitive device.

It is another object of the present invention to prevent popcorning anddelamination of a moisture sensitive device that contains lead-freeplating materials.

It is still another object of the present invention to provide animproved adhesion promoting composition for copper and copper alloysubstrates.

It is still another object of the present invention to provide a stableadhesion promoting composition for various copper alloy substrates.

To that end, in a preferred embodiment, the present invention relatesgenerally to a process for improving the adhesion of polymeric materialsto metal surfaces, particularly copper and copper alloy surfaces, themethod comprising the steps of:

-   -   1) Contacting the metal surface with an adhesion-promoting        composition comprising:        -   a) an oxidizer;        -   b) an acid;        -   c) a corrosion inhibitor; and        -   d) an organic phosphonate; and    -   2) thereafter bonding the polymeric material to the metal        surface.

The inventor has found that the foregoing process improves the adhesionof metal surfaces to the polymeric materials, particularly when themetal surfaces comprise copper or copper alloys. The process proposed isparticularly suited to the production of silicon semiconductorintegrated circuits and similar microelectronic circuits that require apackage that both encases the circuit and provides an electricalinterconnection to external circuitry.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates generally to an improved adhesionpromoting composition for use in the leadframe industry formicroroughening metal leadframes prior to bonding the leadframe with aplastic encapsulating mold material. The adhesion promoting compositionsdescribed herein are usable in the leadframe industry to roughen copperand copper alloy substrates, which then enhances the adhesion of thesubsequently applied die attach resin or other polymeric material to theleadframe. Examples of suitable copper alloys include, but are notlimited to, C194 alloy (available from Olin Corporation) and C7025 alloy(available from Fisk Alloy Conductors, Inc.)

C194 is a high performance copper alloy used for a variety ofapplications including automotive and electrical connectors,semiconductor lead-frames, sockets, and mass terminations and combinesgood electrical and thermal conductivity with high strength, goodsolderability, and plateability. C194 has a nominal composition of 2.4%iron, 0.03% phosphorus, and 0.1% Zinc, with the balance copper.

Alloy C7025 (available from Fisk Alloy Conductors, Inc.) is a highperformance alloy that combines strength, conductivity, formability andstress relaxation resistance into a unique set of properties. The alloyis a thermally aged material and achieves its properties by combinationsof cold work and heat treatments, all of which is done at the mill. Thealloy's high strength and conductivity combined with its formability andstress relaxation properties make C7025 an excellent electronic alloy,particularly in high temperature environments.

The present invention relates generally to a chemical microrougheningprocess for enhancing the bond of leadframes to a subsequently applieddie attach resin or other resinous or polymeric material used in ICmanufacturing, thereby improving the MSL performance of the integratedcircuit packages utilizing leadframes with lead-free plating materials.The process proposed herein provides optimum adhesion between themetallic and polymeric surfaces.

By virtue of the thermally resistant coating that is applied during theprocess, the substrate oxidation which would normally occur and thatwould adversely affect resin adhesion is prevented from occurring duringthe thermal curing of the die attach resin.

The performance target is moisture sensitivity level 1 (MSL-1) which, asdiscussed above, essentially means that the treated component shouldretain good adhesion even under high humidity conditions which wouldnormally allow water ingress between the die attach resin and thesubstrate, causing failure (i.e., popcorning or delamination).

In a preferred embodiment, the adhesion promoting composition of thepresent invention is usable for microroughening a leadframe comprisingC194 or other similar alloy, which have a high iron content. During useof adhesion promoting compositions of the prior art, it was found thatthe iron content in the working bath rises, which then causes asignificant increase in decomposition of the peroxide and organiccomponents present in the bath (via Fenton's reaction). This in turnseverely limits the useful working life of the bath and/or necessitatesan unacceptably high add back regime for successful maintenance. Otherissues include organic sludge production which introduces obviousapplication issues.

The inventors of the present invention have found that the addition ofvarious organic phosphonates to the adhesion promoting compositionovercomes these noted deficiencies.

More specifically, the present invention relates generally to anadhesion promoting composition for treating a copper or copper alloysubstrate, the adhesion promoting composition comprising:

-   -   a) an oxidizer;    -   b) an acid;    -   c) a corrosion inhibitor;    -   d) an organic phosphonate;    -   e) optionally, a source of halide ions;    -   f) optionally, adhesion enhancing species, which species are        selected from the group consisting of molybdates, tungstates,        tantalates, niobates, vanadates, isopoly or heteropoly acids of        molybdenum, tungsten, tantalum, niobium, vanadium, and        combinations of any of the foregoing; and    -   g) optionally, a water soluble polymer.

The inventor herein has found that the adhesion between a metal surfaceand a polymeric material is enhanced by contacting the metal surfacewith the adhesion-promoting composition prior to bonding the polymericmaterial to the metal surface.

The present invention also relates generally to a process for increasingthe adhesion of a polymeric material to a metal surface, said processcomprising:

-   -   1) contacting the metal surface with an adhesion-promoting        composition comprising:        -   a) an oxidizer;        -   b) an acid;        -   c) a corrosion inhibitor;        -   d) an organic phosphonate;        -   e) optionally, a source of halide ions;        -   f) optionally, adhesion enhancing species, which species are            selected from the group consisting of molybdates,            tungstates, tantalates, niobates, vanadates, isopoly or            heteropoly acids of molybdenum, tungsten, tantalum, niobium,            vanadium, and combinations of any of the foregoing; and        -   g) optionally, a water soluble polymer; and    -   2) thereafter bonding the polymeric material to the metal        surface.

The inventor has found that the proposed adhesion-promoting compositionproduces an improved micro-roughened conversion-coated surface upon themetal. The surface produced is particularly suited to bonding withpolymeric materials in that significantly increased adhesion values areachieved as compared to a non-treated metal surface. In addition theconversion coated (treated) metal surface maintains the increasedadhesion over time and decreases the likelihood of any unwantedreactions occurring over time between the metal and the polymericmaterial.

The process proposed is particularly suited to the manufacture ofsemiconductor integrated circuits. Thus, in this application, the metalleadframe (usually copper or copper alloy) is first cleaned with analkaline cleaner to remove dirt and oil. Thereafter, the cleanedleadframe is treated with the adhesion-promoting composition proposedherein. After treatment, followed by water rinsing and drying, theleadframe is bonded together with polymeric materials such as a dieattach resin or other polymeric material used in IC manufacturing.

The metal surface to be treated may comprise a variety of metals such ascopper, copper alloys, nickel and iron. However, the process of theinvention produces the best results when the metal surfaces comprisecopper or copper alloys. The process of the invention is particularlysuited to copper alloys that contain iron such as the C194 and otheralloys having a high iron content.

Polymeric materials include polymeric mold materials which are typicallythermosettable plastic compounds including, for example, novolac,anhydride, biphenyl, and multiaromatic resins, by way of example and notlimitation. The process described herein has been found to improve themoisture sensitivity as well as the MSL of integrated circuit packagesmade of these mold compounds.

The oxidizer used in the adhesion-promoting composition may comprise anyoxidizer which is capable of oxidizing the metal surface in the matrixof the adhesion-promoting composition. The inventors have found hydrogenperoxide and persulfates to be particularly preferred oxidizers for usein the process of the invention, with hydrogen peroxide being the mostpreferred oxidizer. The concentration of the oxidizer in theadhesion-promoting composition may range from 0.5 to 120 grams per literbut is preferably from 2 to 60 grams per liter and is most preferablyfrom 3 to 30 grams per liter.

The acid utilized in the adhesion-promoting composition may be any acidwhich is stable in the matrix, however, the inventors have found mineralacids to be particularly preferred. Sulfuric acid is especiallypreferred. The concentration of the acid in the adhesion-promotingcomposition may range from 1 to 360 grams per liter but is preferablyfrom 20 to 110 grams per liter.

The corrosion inhibitor used in the adhesion-promoting composition is acompound which effectively reacts with the metal surface to form aprotective complex layer. Preferred corrosion inhibitors are selectedfrom the group consisting of triazoles, benzotriazoles, tetrazoles,imidazoles, benzimidazoles and mixtures of the foregoing. Benzotriazolesare particularly preferred. The concentration of the corrosion inhibitorin the adhesion-promoting composition may range from 0.1 to 50 grams perliter but is preferably from 0.2 to 5 grams per liter.

The composition also comprises an organic phosphonate. The inventors ofthe present invention have found that the use of an organic phosphonatein the composition stabilizes the oxidizer and organic componentspresent in the bath and prevents decomposition of the components. Thisin turn increases the working life of the bath and alleviates organicsludge production. Suitable organic phosphonates include1-hydroxy-1,1-ethylidene diphosphonate commercially available as DEQUEST2010 (available from Thermphos International) as well as further similarphosphonate compounds. By way of non-limiting example further usefulperoxide stabilizers include: amino tri(methylene-phosphonic acid)available as DEQUEST 2000 and DEQUEST 2000LC; aminotri(methylene-phosphonic acid) pentasodium salt available as DEQUEST2006; 1-hydroxyethylene-1,1,-diphosphonic acid commercially available asDEQUEST 2010; 1-hydroxyethylene-1,1,-diphosphonic acid tetrasodium saltavailable as DEQUEST 2016 and DEQUEST 2016D; ethylene diaminetetra(methylene phosphonic acid) available as DEQUEST 2041; ethylenediamine tetra(methylene phosphonic acid) pentasodium salt available asDEQUEST 2046; hexamethylenediamine tetra(methylene phosphonic acid)potassium salt available as DEQUEST 2054; diethylenetriaminepenta(methylene phosphonic acid) available as DEQUEST 2060S;diethylenetriamine penta(methylenephosphonic acid) trisodium saltavailable as DEQUEST 2066A; diethylenetriamine penta(methylenephosphonicacid) pentasodium salt available as DEQUEST 2066; diethylenetriaminepenta(methylene phosphonic acid) pentasodium salt commercially availableas DEQUEST 2066C2; bis-hexamethylene triaminepenta(methylenephosphonicacid) chloride salt commercially available as DEQUEST 2090A2-phosphonobutane-1,2,4-tricarboxylic acid commercially available asDEQUEST 7000, tetrasodium salt of 1-hydroxy ethyliden (1,1-diphosphonicacid) commercially available as DEQUEST SPE 9528, as well as othermaterials sold under the DEQUEST tradename, particularly DEQUEST 2086,DEQUEST 3000S, as well as DEQUEST 6004 (all available from ThermphosInternational),

The organic phosphonate is typically present in the composition at aconcentration of between about 0.1 to 120 grams per liter, morepreferably at a concentration of between about 0.1 to 50 grams perliter. Preferably the organic phosphonate is 1-hydroxy-1,1-ethylidenediphosphonate, which is present in the composition at a concentration ofbetween about 0.1 to 120 grams per liter, more preferably at aconcentration of between about 3 to 30 grams per liter.

Optionally, the composition may also include a source of adhesionenhancing species, which can be any material that will supply speciesselected from the group consisting of molybdates, tungstates,tantalates, niobates, vanadates and mixtures thereof to the adhesionpromoting composition. Such sources include alkali metal salts ofmolybdates, tungstates, tantalates, niobates, vanadates and mixturesthereof such as sodium (or potassium) molybdate, tungstate, niobate orvanadate, and heteropoly acids or isopoly acids of molybdenum, tungsten,tantalum, niobium or vanadium. Thus, molybdates or tungstates whichinclude heteroatoms such as phosphorous, silicon, cobalt, manganese andtungsten are suitable. Preferred sources include iso and heteropolyacids of molybdenum, tungsten, niobium, vanadium and mixturesthereof such as molybdic acid, vanadic acid and tungstic acid. The mostpreferred source of adhesion enhancing species is molybdic acid. Theconcentration of adhesion enhancing species in the adhesion promotingcomposition may range from 1 mg/l to 500 mg/l (based on the adhesionenhancing ion content) but is preferably from 5 mg/l to 200 mg/l. Theadhesion-enhancing species may be utilized with or without thebenzotriazole with the electron withdrawing group in the 1-position.

Optionally, the adhesion-promoting composition may also comprise a watersoluble polymer. If used, the water soluble polymer is preferably not awetter or surfactant but is instead a water soluble homopolymer orcopolymer of low molecular weight water soluble monomers. Mostpreferably, the water soluble polymer is a polymer of ethylene oxide, anethylene oxide-propylene oxide copolymer, polyethylene glycols,polypropylene glycols or polyvinyl alcohols. Among the most preferredare the polymers of ethylene oxide, or polyethylene glycols sold by theUnion Carbide Company under the tradename Carbowax. The inventors havefound Carbowax 750 and Carbowax MPEG 2000 to be particularly useful.Also particularly useful are the ethylene oxide polymers or ethyleneoxide-propylene oxide copolymers sold by the BASF or Company under thePluronic tradename. The concentration of the water soluble polymer inthe adhesion-promoting composition can range from 0.2 to 15 grams perliter, but is preferably from 3 to 6 grams per liter.

The adhesion promoting composition also optionally, but preferably,contains a source of halide ions. The source of halide ions may be anycompound which would provide halide ions in the matrix of theadhesion-promoting composition. Preferably, the source of halide ionsare alkaline metal salts such as sodium chloride or potassium chloride,oxohalides such as sodium chlorate or potassium chlorate, or halidebearing mineral acids such as hydrochloric acid. Preferably the sourceof halide ions provides chloride ions to the adhesion-promotingcomposition. The concentration of the source of halide ions in theadhesion-promoting composition may range from 0.5 to 500 milligrams perliter but is preferably from 1 to 12 milligrams per liter, all based onhalide ion content.

The metal leadframe can be treated with the adhesion-promotingcomposition of the composition in a variety of ways, includingimmersion, spray, or flood. The temperature of the adhesion-promotingcomposition during treatment may range from 80° F. to 150° F. but ispreferably from 90° F. to 120° F. The treatment time will vary dependingupon the temperature and method of treatment but may range from 15seconds to 15 minutes and is preferably from 1 to 2 minutes.

The inventors have found that the use of the improved bath compositiondescribed herein provides an improved bond between the copper leadframeand the die attach resin or other polymeric material and at leastsubstantially eliminates popcorning and delamination of the IC device.

What is claimed is:
 1. A process for increasing the adhesion of apolymeric material to a metal surface, the process comprising: a)contacting the metal surface with an adhesion promoting compositioncomprising: 1) an oxidizer; 2) an inorganic acid; 3) a corrosioninhibitor; and 4) an organic phosphonate; and thereafter b) bonding thepolymeric material to the metal surface.
 2. The method according toclaim 1, wherein the oxidizer is hydrogen peroxide.
 3. The methodaccording to claim 1, wherein the inorganic acid is a mineral acid. 4.The method according to claim 1, wherein the mineral acid is sulfuricacid.
 5. The method according to claim 1, wherein the corrosioninhibitor is selected from the group consisting of triazoles,benzotriazoles, tetrazoles, imidazoles, benzimidazoles and combinationsof one or more of the foregoing.
 6. The method according to claim 5,wherein the corrosion inhibitor is a benzotriazole.
 7. The methodaccording to claim 1, wherein the organic phosphonate is selected fromthe group consisting of 1-hydroxy-1,1-ethylidene diphosphonate, aminotri(methylene-phosphonic acid), amino tri(methylene-phosphonic acid)pentasodium salt, 1-hydroxyethylene-1,1,-diphosphonic acid,1-hydroxyethylene-1,1,-diphosphonic acid tetrasodium salt, ethylenediamine tetra(methylene phosphonic acid), ethylene diaminetetra(methylene phosphonic acid) pentasodium salt, hexamethylenediaminetetra(methylene phosphonic acid) potassium salt, diethylenetriaminepenta(methylene phosphonic acid), diethylenetriaminepenta(methylenephosphonic acid) trisodium salt, diethylenetriaminepenta(methylenephosphonic acid) pentasodium salt, diethylenetriaminepenta(methylene phosphonic acid) pentasodium salt; bis-hexamethylenetriaminepenta(methylenephosphonic acid) chloride salt,2-phosphonobutane-1,2,4-tricarboxylic acid, tetrasodium salt of1-hydroxy ethyliden (1,1-diphosphonic acid) and combinations of one ormore of the foregoing.
 8. The method according to claim 7, wherein theorganic phosphonate is 1-hydroxy-1,1-ethylidene diphosphonate.
 9. Themethod according to claim 7, wherein the organic phosphonate is presentin the composition at a concentration of between about 0.1 to 120 gramsper liter.
 10. The method according to claim 8, wherein the1-hydroxy-1,1-ethylidene diphosphonate is present in the composition ata concentration of between about 0.1 to 120 grams per liter.
 11. Themethod according to claim 1, wherein the metal surface comprises acopper or copper alloy surface.
 12. The method according to claim 11,wherein the metal surface is a copper alloy leadframe and the polymericmaterial is a die attach resin.
 13. The method according to claim 12,wherein the copper alloy has a high iron content.